U.S. patent application number 13/103332 was filed with the patent office on 2012-11-15 for high performance optical polarization diversity circuit.
Invention is credited to Long Chen, Christopher Doerr.
Application Number | 20120288229 13/103332 |
Document ID | / |
Family ID | 47141950 |
Filed Date | 2012-11-15 |
United States Patent
Application |
20120288229 |
Kind Code |
A1 |
Doerr; Christopher ; et
al. |
November 15, 2012 |
HIGH PERFORMANCE OPTICAL POLARIZATION DIVERSITY CIRCUIT
Abstract
An optical device includes an input/output optical coupler, a
waveguide and a waveguide fragment. The optical coupler is
configured to separate a received optical signal into first and
second signal components. The waveguide is connected to the optical
coupler and configured to propagate the first signal component via
a first propagation mode. The waveguide fragment is located
adjacent to the first waveguide and is configured to couple light
from the first waveguide that propagates therein by a different
second propagation mode.
Inventors: |
Doerr; Christopher;
(Middletown, NJ) ; Chen; Long; (Matawan,
CN) |
Family ID: |
47141950 |
Appl. No.: |
13/103332 |
Filed: |
May 9, 2011 |
Current U.S.
Class: |
385/29 ;
29/874 |
Current CPC
Class: |
G02B 2006/12123
20130101; G02B 6/34 20130101; G02B 6/2813 20130101; G02B 6/4213
20130101; G02B 6/124 20130101; G02B 6/126 20130101; Y10T 29/49204
20150115; G02B 6/125 20130101; G02B 2006/12116 20130101; G02B
2006/12061 20130101; G02B 2006/12147 20130101; G02B 2006/1215
20130101; G02B 6/30 20130101; G02B 6/12004 20130101 |
Class at
Publication: |
385/29 ;
29/874 |
International
Class: |
G02B 6/26 20060101
G02B006/26; G02B 6/24 20060101 G02B006/24 |
Claims
1. An optical device, comprising: a input/output optical coupler
configured to separate a first received optical signal into first
and second signal components; a first waveguide connected to said
input/output optical coupler and configured to propagate said first
signal component via a first propagation mode; a first waveguide
fragment adjacent said first waveguide, said first waveguide
fragment being configured to couple light from said first waveguide
that propagates by a different second propagation mode.
2. The optical device of claim 1, further comprising a second
waveguide fragment located adjacent said first waveguide and
configured to couple light from said first waveguide that
propagates by said second propagation mode.
3. The optical device of claim 1, wherein said input/output optical
coupler is a first waveguide grating, and further comprising a
second waveguide connected to said first waveguide grating, said
second waveguide being about perpendicular to said first waveguide
at said first waveguide grating and configured to propagate said
second optical signal via said first propagation mode.
4. The optical device of claim 3, further comprising: a second
waveguide grating configured to separate a second received optical
signal into third and fourth signal components; a third waveguide
connected to said second waveguide grating and configured to
propagate said third signal component via said first propagation
mode; a second waveguide fragment located adjacent said third
waveguide, said second waveguide fragment being configured to
couple light from said third waveguide that propagates by said
second propagation mode.
5. The optical device of claim 4, further comprising a 2.times.2
multi-mode interference coupler connected to said first and third
waveguides.
6. The optical device of claim 1, wherein said input/output optical
coupler is a waveguide grating, and further comprising a second
waveguide connected to said grating coupler, said second waveguide
being about parallel to said first waveguide at said waveguide
grating and configured to propagate said second signal component
via said second propagation mode.
7. The optical device of claim 3, further comprising a third
waveguide adjacent to said second waveguide and configured to
couple said second signal component via said second propagation
mode.
8. The optical device of claim 3, wherein said first waveguide
grating is one of a plurality of waveguide gratings configured to
optically couple to a corresponding plurality of optical cores of a
multi-core fiber.
9. The optical device of claim 1, wherein said first input/output
optical coupler is a two-dimensional pattern grating coupler.
10. The optical device of claim 1, wherein said first waveguide is
a waveguide of a coherent receiver.
11. The optical device of claim 1, wherein said first propagation
mode is transverse-electric and said second propagation mode is
transverse-magnetic.
12. The optical device of claim 1, wherein said first propagation
mode is about orthogonal to said second propagation mode.
13. A method, comprising: forming an input/output optical coupler
configured to separate a first received optical signal into first
and second signal components; forming a first waveguide connected
to said input/output optical coupler and configured to propagate
said first signal component via a first propagation mode; forming a
first waveguide fragment adjacent said first waveguide, said first
waveguide fragment being configured to couple light from said first
waveguide that propagates by a different second propagation
mode.
14. The method of claim 13, wherein said input/output optical
coupler is a waveguide grating, and further comprising forming a
second waveguide connected to said waveguide grating, said second
waveguide being about parallel to said first waveguide at said
waveguide grating and configured to propagate said second signal
component via said second propagation mode.
15. The method of claim 14, further comprising forming a third
waveguide adjacent to said second waveguide and configured to
couple said second signal component via said second propagation
mode.
16. The method of claim 13, wherein said input/output optical
coupler is a waveguide grating, and further comprising forming a
second waveguide connected to said waveguide grating, said second
waveguide being about perpendicular to said first waveguide at said
waveguide grating and configured to propagate said second optical
signal via said first propagation mode.
17. The method of claim 13, wherein said input/output optical
coupler is one of a plurality of waveguide gratings configured to
optically couple to a corresponding plurality of optical cores of a
multi-core fiber.
18. The method of claim 13, wherein said input/output optical
coupler is a first waveguide grating and further comprising:
forming a second waveguide grating configured to separate a second
received optical signal into third and fourth signal components;
forming a third waveguide connected to said second waveguide
grating and configured to propagate said third signal component via
said first propagation mode; forming a second waveguide fragment
adjacent said third waveguide, said second waveguide fragment being
configured to couple light from said third waveguide that
propagates by said second propagation mode.
19. The method of claim 18, further comprising connecting a
2.times.2 multi-mode interference coupler to said first and third
waveguides.
20. The method of claim 13, wherein said input/output optical
coupler is a two-dimensional pattern grating coupler.
21. The method of claim 13, wherein said first waveguide is a
waveguide of a coherent receiver.
22. The method of claim 13, wherein said first propagation mode is
transverse-electric and said second propagation mode is
transverse-magnetic.
23. The method of claim 13, wherein said first propagation mode is
about orthogonal to said second propagation mode.
Description
TECHNICAL FIELD
[0001] This application is directed, in general, to optical devices
and methods of manufacturing and using optical devices.
BACKGROUND
[0002] This section introduces aspects that may help facilitate a
better understanding of the invention(s). Accordingly, the
statements of this section are to be read in this light and are not
to be understood as admissions about what is in the prior art or
what is not in the prior art.
[0003] A coherent optical-detection scheme is capable of detecting
not only the amplitude of an optical signal, but also the signal's
polarization and phase. These capabilities make coherent optical
detection compatible with polarization-division multiplexing (PDM)
and with the use of spectrally efficient modulation formats, such
as quadrature amplitude modulation (QAM) and phase-shift keying
(PSK) in its various forms (e.g., differential PSK (DPSK) and
differential quadrature PSK (DQPSK). Compared to incoherent
detectors, coherent optical detectors offer relatively easy
wavelength tunability, good rejection of interference from adjacent
channels in dense wavelength-division-multiplexing (DWDM) systems,
linear transformation of the electromagnetic field into an
electrical signal for effective application of modern digital
signal processing techniques, and an opportunity to use
polarization-division multiplexing. As a result, coherent optical
detectors are currently being actively developed.
SUMMARY
[0004] One aspect provides an optical device that includes an
optical input/output coupler, a waveguide and a waveguide fragment.
The optical coupler is configured to separate a received optical
signal into first and second signal components. The waveguide is
connected to the optical coupler and configured to propagate the
first signal component via a first propagation mode. The waveguide
fragment is located adjacent the first waveguide and is configured
to couple light from the first waveguide that propagates therein by
a different second propagation mode.
[0005] Another aspect provides a method. The method includes
forming an input/output optical coupler, a waveguide and a
waveguide fragment. The optical coupler is configured to separate a
received optical signal into first and second signal components.
The waveguide is connected to the optical coupler and configured to
propagate the first signal component via a first propagation mode.
The waveguide fragment is located adjacent to the first waveguide
and is configured to couple light from the first waveguide that
propagates therein by a different second propagation mode.
BRIEF DESCRIPTION
[0006] Reference is made to the following descriptions taken in
conjunction with the accompanying drawings, in which:
[0007] FIG. 1 illustrates waveguide fragment according to one
embodiment of the disclosure, wherein the waveguide fragment is
configured to filter TM noise from an optical waveguide, such as
used in the receiver 200 of FIG. 2;
[0008] FIGS. 2A-2C illustrate a photonic integrated circuit having
a receiver configured to demodulate a polarization division
multiplexed quadrature phase shift keyed optical signal;
[0009] FIG. 3 illustrates a multicore fiber switching network that
includes waveguide fragments placed to filter TM noise from some
waveguides;
[0010] FIG. 4 illustrates a waveguide grating according to one
embodiment that may be used in the multicore fiber network of FIG.
3;
[0011] FIGS. 5A and 5B respectively illustrate aspects of a TM-mode
filter and a TE-mode filter provided in the multicore fiber
switching network of FIG. 3; and
[0012] FIG. 6 illustrates a method according to one embodiment,
e.g. of forming optical devices of the disclosure, e.g. as
described by FIGS. 1-5.
DETAILED DESCRIPTION
[0013] This disclosure benefits from the recognition that unwanted
polarization modes of an optical signal may be effectively coupled
from a primary optical waveguide to a secondary optical waveguide
fragment placed near the waveguide. The unwanted polarization mode
may thereby be removed, e.g. filtered, from the primary waveguide,
thereby increasing the fidelity of later processing of the optical
signal. Such filtering may, e.g. reduce an input signal-to-noise
ratio required by an optical receiver to achieve a desired bit
error rate.
[0014] FIG. 1 illustrates a plan view of a propagation mode filter
100 according to one embodiment of the disclosure. The mode filter
100 includes a primary waveguide 110 and a secondary waveguide 120.
The secondary waveguide 120 includes a coupled portion 130 and an
optional redirecting portion 140. The coupled portion 130 runs
adjacent and about parallel to the primary waveguide 110 over a
length L. The redirecting portion 140, when present, substantially
deviates from parallel to the primary waveguide 110.
[0015] The primary waveguide 110 is a waveguide that propagates, or
is configured to propagate, an optical signal to the vicinity of
the secondary waveguide 120. The secondary waveguide 120 is a
waveguide configured to couple optical energy from the primary
waveguide 110. The secondary waveguide 120 may be, and is
illustrated as, a waveguide fragment, and may be referred to herein
as such. Herein a waveguide fragment is a waveguide structure that
has no direct source or destination. Having no direct source means
that there is no source of optical energy to the waveguide fragment
other than light coupled indirectly from another waveguide, e.g.
from the primary waveguide 110, such as via an evanescent wave.
Having no direct destination means that the waveguide fragment is
not connected to any device that is configured to manipulate
optical energy propagated by the waveguide fragment, e.g. a switch,
coupler or photodiode. In an illustrative embodiment energy coupled
to the secondary waveguide 120 from the primary waveguide 110 is
incoherently scattered to the surrounding medium.
[0016] The primary waveguide 110 and the secondary waveguide 120
may be, e.g. planar or ridge waveguide structures. They may be
formed of any suitable optical medium over any suitable substrate.
For example the optical medium may be, e.g. semiconductors such as
silicon or InGaAsP, or dielectrics such as Si.sub.3N.sub.4 or
SiO.sub.2. The discussion herein may refer to the medium as silicon
without limitation. The substrate may include a semiconductor
wafer, e.g. a silicon wafer, or a portion of a semiconductor wafer.
An optical isolation layer, e.g. SiO.sub.2, may be located between
the substrate and the waveguides 110, 120. A cladding layer, e.g.
SiO.sub.2, may be formed over the waveguides 110, 120.
[0017] In various embodiments the primary waveguide 110 may
propagate an optical signal 150 in a first and a different second
propagation mode. Optionally the different propagation modes may be
about orthogonal to each other. For example the propagation modes
may be transverse-electric (TE) and transverse-magnetic (TM) modes.
The secondary waveguide 120 may be configured to couple more
efficiently to the TE mode or to the TM mode of the signal 150. In
an illustrative embodiment the secondary waveguide 120 is
configured to couple more efficiently to the TM mode. Thus, for
example, both TE and TM light of the signal 150 may initially
propagate within the primary waveguide 110. The TM mode may then
couple to the secondary waveguide 120 thereby transferring the
energy associated with the TM mode to the secondary waveguide 120.
The transferred energy is optionally directed away from the primary
waveguide 110 and is dissipated to the medium surrounding the
secondary waveguide 120. The secondary waveguide 120 and the
portion of the primary waveguide 110 coupled thereto thereby
function as a propagation mode filter to remove energy associated
with the TM mode of the signal 150 from the primary waveguide 110.
Embodiments illustrating the utility of this function are described
below.
[0018] In a nonlimiting example, the primary waveguide 110 is
formed from silicon and is configured to propagate a 1.55 .mu.m
optical signal (C band). The primary waveguide 110 may have a width
W.sub.1 of about 430 nm and thickness of about 220 nm. The
secondary waveguide 120 has a width W.sub.2 that may be about equal
to W.sub.1, e.g. also about 430 nm, and a thickness of about 220
nm. The coupled portion 130 runs about parallel to the primary
waveguide 110 over a distance L of at least about 2.4 .mu.m with a
spacing D about equal to 450 nm. In this example the intensity of
the TM mode in the primary waveguide 110 may be reduced by at least
about 10 dB. In some embodiments, not shown, multiple secondary
waveguides, e.g. waveguide fragments, may be located next to the
primary waveguide 110 in a sequential manner to increase the
attenuation of the TM mode therein. For example, N instances of the
secondary waveguide 120 may attenuate the TM mode by about N*10
dB.
[0019] As mentioned previously, the redirecting portion 140 is
optional. It is expected, however, that the benefit of the filter
100 will be greater when the energy coupled to the secondary
waveguide 120 is redirected away from the primary waveguide 110.
Furthermore, it is expected that performance of the filter 100 will
be further enhanced when the end of the secondary waveguide 120 is
far enough from the primary waveguide 110 that light scattered from
the end does not significantly couple back into the primary
waveguide 110. This distance is expected to depend on factors such
as wavelength and intensity of the signal 150, but under some
conditions 2-3 .mu.m is expected to be sufficient.
[0020] When the redirecting portion 140 is present, it is
advantageous that this portion provides a smoothly varying
direction with a radius of curvature large enough to guide the
light within the secondary waveguide 120. In some embodiments, a
bend radius R of at least about 10 .mu.m is sufficient to minimize
scattering and effectively guide light therethrough. However,
embodiments in which the redirecting portion 140 is other than a
smoothly varying path, e.g. a sharp bend, are within the scope of
the disclosure. Contemplated embodiments include those in which the
one or both of the waveguides 110, 120 includes a bend immediately
before or after the coupled portion 130. In the embodiment
illustrated in FIG. 1, the bends are about 90.degree., though the
bend or bends are not limited thereto.
[0021] Two embodiments are now presented that include one or more
instances of coupled waveguides configured to transfer energy of
one propagation mode from a primary to a secondary waveguide. While
various aspects of these embodiments and benefits provided by such
a configuration are described in detail, such descriptions should
not be taken to limit the scope of the disclosure to such described
uses.
[0022] FIG. 2A schematically shows a coherent receiver 200, formed
as a photonic integrated circuit (PIC), according to one embodiment
of the invention. More specifically, FIG. 2A shows a top view of
various components of the receiver 200 in a layout according to one
embodiment. The receiver 200 includes two pairs of crossing
waveguides. Each pair of waveguides forms a waveguide crossing at
the location of intersection. A first unreferenced crossing
waveguide pair includes tapered waveguides 205, 210 and a waveguide
grating 230. A second unreferenced crossing waveguide pair includes
tapered waveguides 255, 260 and a waveguide grating 250. The
waveguide gratings are illustrative embodiments of optical
input/output couplers that may be used in various embodiments of
the disclosure. Herein and in the claims, an input/output coupler
is a coupler configured to receive an optical signal from a source
external to the PIC 200, or to provide a signal to a destination
external to the PIC 200. In various embodiments the waveguide
gratings 230, 250 couple to the core of a fiber optic cable (not
shown). As discussed further below the waveguide grating 230 may
receive a modulated optical signal, e.g. modulated by QPSK to carry
digital information. The waveguide grating 250 may receive an
unmodulated optical signal, e.g. a local oscillator (LO).
[0023] FIG. 2B shows an enlarged top view of the waveguide grating
230, which is also representative of the waveguide grating 250. In
a representative embodiment, the waveguide grating 230 comprises a
plurality of cavities, pillars, and/or holes 240 etched into or
formed on an upper surface of a ridge waveguide to form a
two-dimensional, rectangular or square pattern. Each of the four
sides of the waveguide grating 230 is connected to a corresponding
one of waveguides 205a, 205b, 210a or 210b. The waveguides 205a-b
are collinear with each other at the grating 230 and orthogonal to
the waveguides 210a-b, which are similarly collinear with each
other. A grating that can be used as the waveguide grating 230 is
disclosed, e.g., in U.S. Pat. No. 7,065,272, which is incorporated
herein by reference in its entirety.
[0024] The waveguide grating 230 serves at least three different
functions in the receiver 200, e.g., those of (1) an optical fiber
coupler, (2) a polarization splitter, and (3) two power splitters,
one for each of two different, e.g. orthogonal, polarizations of
the optical input signal. More specifically, if the grating 230 is
physically abutted with a single-mode optical fiber, e.g., oriented
orthogonally with respect to the upper surface of the grating
(i.e., perpendicular to the plane of FIGS. 2A-2C), then light from
the optical fiber will couple from the optical fiber into
waveguides 205a-b and 210a-b, hence the optical fiber coupler
functionality of grating 230. If the light in the optical fiber has
two polarization components, e.g., an X-polarization component and
a Y-polarization component, then the X-polarization component will
couple into waveguides 210a-b and the Y-polarization component will
couple into waveguides 205a-b, hence the polarization-splitter
functionality of grating 230. The coupled optical power of the X
polarization will be divided substantially evenly between
waveguides 210a-b, hence the power-splitter functionality of
grating 230 for the X polarization. Similarly, the coupled optical
power of the Y polarization will be divided substantially evenly
between waveguides 205a-b, hence the power-splitter functionality
of grating 230 for the Y polarization. Additional aspects of the
operation of the grating are disclosed, e.g. in PCT Application No.
WO 2010/107439 A1, which is incorporated herein by reference in its
entirety.
[0025] Note that if the optical input signal has a single carrier
wavelength, then all three of the above-described functions apply
to the signal component having that carrier wavelength. If the
optical input signal has multiple carrier wavelengths, then each of
the signal components corresponding to different carrier
wavelengths is subjected to each of the three above-described
functions. The fiber-optic coupling efficiency of the grating 230
can be optimized for any selected wavelength or a range or
wavelengths by using a corresponding appropriate pattern of
cavities or holes 240. For example, the above-cited U.S. Pat. No.
7,065,272 discloses patterns that can be used for efficiently
coupling light having wavelengths between about 1500 nm and about
1600 nm. One skilled in the art will appreciate that, to obtain a
grating suitable for efficient coupling of other wavelengths, the
disclosed patterns can be modified, e.g., by appropriately changing
the periodicity of cavities or holes in the grating.
[0026] With continuing reference to FIG. 2B, in an embodiment holes
240p at the perimeter of the waveguide grating 230 are formed
differently from holes 240i formed in the interior of the grating
230. For example, in the nonlimiting case that the grating 230 is
configured to receive 1.55 .mu.m light, the holes 240 are formed
with a lattice spacing of about 580 nm. The holes 240p have a
diameter of about 350 nm and a depth of about 55 nm. The holes 240i
have a diameter of about 290 nm and a depth of about 120 nm. It is
believed that this configuration of the holes 240 provides
apodization of the exponential field profile of the grating 230 and
reduces the grating back-reflection.
[0027] FIG. 2C illustrates the pair of waveguides 205, 210 in
isolation, including the waveguide grating 230. The waveguides 205,
210 and the waveguide grating 230 are referred to herein as
receiving section 235. A first portion of the X polarization of the
received optical signal is directed to a port 245a of the receiving
section 235, and a second portion of the X polarization is directed
to a port 245b. Similarly, a first portion of the Y polarization of
the received optical signal is directed to a port 245c, and a
second portion of the Y polarization is directed to a port 245d. In
various embodiments the first and second portions of the X
polarization are about equal in power, and the first and second
portions of the Y polarization are about equal in power. However,
the power of the X polarization may be different from the power of
the Y polarization.
[0028] In a representative embodiment of the receiver 200, the X
polarization from an abutted optical fiber efficiently couples into
and propagates along each of waveguides 210a-b as a corresponding
transverse electric (TE) waveguide mode. The X polarization couples
into waveguides 205a-b relatively inefficiently, and this coupling
is negligible for all practical purposes. Similarly, the Y
polarization from the abutted optical fiber couples efficiently
into each of waveguides 205a-b as a corresponding TE waveguide mode
and negligibly into waveguides 210a-b.
[0029] Typically the energy coupled into a TM waveguide mode of the
waveguides 205a-b and 210a-b is also negligible. However, the
inventors have found that in some demanding applications, such as
for the receiver 200, energy propagating by the TM mode, sometimes
referred to herein as TM noise, may be great enough to adversely
limit the performance of the receiver 200. TM noise may be
generated, e.g. by non-coherent scattering processes within the
grating 230 and/or the waveguides 205a-b and 210a-b.
[0030] The inventors have further determined that the adverse
effects of the TM noise may be substantially reduced by placing an
instance of a waveguide fragment 220 near each of the ports 245a-d,
where each of the waveguide fragments 220 is configured as
described in FIG. 1 to remove TM energy from the signals
propagating within the waveguides 205a-b and 210a-b. In some
embodiments multiple auxiliary waveguides are placed in series
along the same waveguide to increase the amount of TM energy
removed.
[0031] Returning to FIG. 2A, the second (unreferenced) receiving
section is configured nominally identically to the first receiving
section 235. The second receiving section includes ports 265a-d.
Corresponding ports of the first and second receiving sections are
coupled to inputs of respective 2.times.2 optical couplers, e.g.
multimode interference (MMI) couplers, 270a-d. Outputs of the
couplers 270a-d are coupled to photodetectors (e.g. PIN
photodiodes) 275a-h, where signals provided by the coupler outputs
are converted to the electrical domain for further processing. In
an embodiment the photodetectors 275a-h are Ge-on-Si vertical PIN
photodiodes with Ge regions having sides of about 0.8 .mu.m.times.8
.mu.m.times.40 .mu.m for low capacitance and thus high speed.
[0032] The waveguide grating 250 may receive an LO selected to
demodulate a PDM QPSK signal received by the waveguide grating 230.
The waveguide grating 250 may couple equal portions of an X
polarization of the LO to waveguides 255a-b and equal portions of a
Y polarization to waveguides 260a-b. The X-polarized portions are
routed to ports 265a-b, and the Y-polarized portions are routed to
ports 265c-d. The path lengths are nominally equal from each
grating coupler of the waveguide gratings 230, 250 to each
associated coupler 270. Thermo-optic phase shifters 280a-b provide
a controllable phase shift to adjust the optical path lengths of
the waveguides 205a, 210a to provide a .pi./2 phase shift to the X
and Y polarizations of the received signal propagating in the
waveguides 205a, 210a.
[0033] In some embodiments each of the diode pairs, e.g. 275a-b, is
connected in series to determine an intensity difference between
the light received by the diodes. However this approach may about
double the capacitance seen at electrical output of the photodiode
pair as compared to a single photodiode, potentially reducing the
maximum operating speed of the receiver. Moreover, the subtracted
photodiode output is typically conditioned by a transimpedance
amplifier (TIA) before further processing, and it is typically more
difficult to make a linear single-ended TIA than it is to make a
linear differential TIA.
[0034] Accordingly, in some embodiments differential TIAs are used
to condition the outputs of the photodiodes 275. Four differential
TIAs may each receive the outputs from each of a pair of photodiode
outputs, e.g. photodetector pairs 275a-b, 275c-d, 275e-f and
275g-h. In such embodiments the electrical signal representing the
output of one photodetector of a pair of photodetectors 275 is
subtracted from the electrical signal representing the output of
the other photodetector of the pair of photodetectors 275 within
the TIA. This configuration is expected to result in higher speed
and more linear amplification than implementations in which
single-ended TIAs are used as described above. In various
embodiments the TIA is external to the PIC of the receiver 200. The
TIA inputs may be wire bonded to bond pads formed on the receiver
200 for this purpose.
[0035] With continued reference to FIG. 2A, for each of the X and Y
polarizations of an optical communication signal applied to the
grating 230, the receiver 200 achieves QPSK demodulation by mixing
an in-phase (I) component and a quadrature (Q) component of each
polarization with the LO to produce I and Q data components for
each polarization. For example, when the X polarization of the
received signal is split, a first half of the received power
propagates to the coupler 270b, and a second half propagates to the
coupler 270c. The second half is subjected to a .pi./2 phase shift
relative to the first half by the thermo-optic shifter 280b. The
first and second halves of the LO X polarization are also directed
to the couplers 270b-c. The coupler 270b produces balanced outputs
for the I component of the X polarization data channel, and the
coupler 270c produces balanced outputs for the Q component of the X
polarization channel. Recovery of the I and Q data components of
the Y polarization data channel occurs analogously, with the I
component output at the coupler 270d and the Q component output at
the coupler 270a.
[0036] In one embodiment, the waveguides 205, 210, optical couplers
270a-b, and photo-detectors 275a-h are all implemented in a
monolithic PIC using integration techniques disclosed, e.g., in
U.S. patent application Ser. No. 12/229,983. Other known
integration techniques may likewise be used.
[0037] It is noted that the use of the waveguide fragment 220 is
not limited to use with optical devices that use waveguide gratings
to separate polarizations of a polarization-division multiplexed
signal. For example, a polarization beam splitter may be used to
separate TE and TM propagation modes of an optical signal
propagating in a planar waveguide. After separation, the TM
component may be rotated to a TE propagation mode. Both signal
components now being TE polarized, subsequent optical processing
may be simplified. See, e.g., Hiroshi Fukuda, et al., "Polarization
Beam Splitter and Rotator for Polarization-Independent Silicon
Photonic Circuit", 4.sup.th IEEE International Conference on Group
IV Photonics (2007), Oct. 15, 2007, INSPEC Accession Number
10089350, incorporated herein by reference in its entirety. A
waveguide fragment within the scope of the disclosure may be used
to filter TM noise from both signal components after the received
TM propagation mode is rotated to TE.
[0038] Turning now to FIG. 3, another embodiment of the disclosure
is illustrate, a multi-core fiber (MCF) switching network 300.
Aspects of the switching network are described in U.S. patent
application Ser. No. 13/012,712 (the '712 application),
incorporated herein in its entirety.
[0039] Input arrays 305a-b of waveguide gratings 310 are each
configured to receive optical signals from a plurality of optical
cores of an MCF (not shown). In the illustrated embodiment, each
array 305 includes seven waveguide gratings 310, so a corresponding
MCF may include seven optical fiber cores. As described in the '712
application, the waveguide gratings 310 are configured to couple
one polarization, e.g. X-polarized light, from a received optical
signal to a TE propagation mode propagating to the right. The
waveguide gratings 310 are similarly configured to couple another
polarization, e.g. Y-polarized light, from the received optical
signal to a TM propagation mode propagating to the left.
[0040] FIG. 4 illustrates an embodiment of the waveguide grating
310. In this embodiment the waveguide gratings 310 are a linear
array of alternating slots and ridges formed in a transition
waveguide 410. The period, width and depth of the slots may be
configured depending on the wavelength of the received optical
signal. In an example embodiment the slots are about 90 nm deep,
about 280 nm wide and have a period of about 560 nm in silicon
waveguides that are about 220 nm thick.
[0041] Referring back to FIG. 3, the TE-polarized light propagates
via waveguides 315 directly to an optical switch network 320. The
switch network 320 is coupled to output arrays 325a-b via
waveguides 330. In various embodiments the mapping of the
waveguides 315 to the waveguides 330 provided by the switch network
320 is arbitrary, meaning that any of the waveguides 315 may be
connected to any of the waveguide 330.
[0042] The TM-polarized light propagates via waveguides 335 and 340
indirectly to an optical switch network 345. The propagation is
indirect by virtue of coupling of TM-mode light between ones of the
waveguides 335 and corresponding ones of the waveguides 340. The
switch network 345 is coupled to the output arrays 325a-b via the
waveguides 340. Again the mapping of the waveguides 335 to the
waveguides 340 provided by the switch network 345 may be
arbitrary.
[0043] In some cases the rightward propagating signals may include
TM noise due to imperfections of the coupling between the MCF core
and the waveguide grating 310. The TM noise may be removed by
waveguide fragments 355. As described previously with respect to
FIGS. 1 and 2A, the waveguide fragments 355 may couple the unwanted
TM light from the waveguides 315, and scatter the light to the
surrounding medium.
[0044] FIG. 5A illustrates one instance of a waveguide fragment 355
adjacent one of the waveguides 315. The waveguide fragment 355 is
located at a distance D.sub.1 from the (primary) waveguide 315. In
the illustrated embodiment the waveguide 315 includes two
90.degree. bends, e.g. an S curve. Thus the waveguide fragment 355
is spaced a distance D.sub.2 from the waveguide 315. While in
various embodiments it may be desirable to limit D.sub.2 to as
small a value as possible to provide a compact waveguide layout, it
is expected that preserving a distance D.sub.2 of at least about
two times D.sub.1, and preferably about three or more times D.sub.1
will reduce coupling of light from the waveguide fragment 355 back
to the waveguide 315 below an acceptable value.
[0045] Similarly the leftward propagating signals may include TE
noise. In this case, each of the waveguides 335 couples to a
corresponding one of the waveguides 340 thereby indirectly coupling
the desired TM light to corresponding waveguide 340. The coupling
is determined by similar considerations as described for the
embodiment of FIG. 1. Thus, for example, TM light is coupled from
the (primary) waveguide 335 to the (secondary) waveguide 340. In
this case, the TM light is the desired signal. The unwanted TE
light remains in the waveguide 335 and is dissipated to the
surrounding medium. The coupled portions of the waveguides 335, 340
therefore operate as polarization mode filters.
[0046] FIG. 5B illustrates a detail of the overlap of one of the
waveguides 335 with a corresponding one of the waveguides 340. The
waveguides 335, 340 overlap with an overlap length L that is
sufficient for a substantial portion, e.g. at least about 71%, of
the TM light to couple from the waveguide 335 to the waveguide 340.
Coupling 71% of the light is equivalent to about a 3 dB coupling
loss. Preferably the overlap provides no greater than about 2 dB
coupling loss (e.g. about 79% coupling), and more preferably the
overlap provides no greater than about 1 dB coupling loss (e.g.
about 89% coupling).
[0047] For example, when the optical carrier propagated by the
waveguides 335, 340 has a wavelength of about 1.55 .mu.m, the
waveguides 335, 340 formed from silicon may have a width W.sub.1 of
about 0.43 .mu.m, the waveguide being laterally separated by a
space D of about 0.45 .mu.m. An overlap length L of about 2.4 .mu.m
and a bend radius of 10 um is expected to couple the TM light with
about a 0.5 dB coupling loss. Those skilled in the pertinent art
are able to determine proper waveguide width and spacing for other
carrier wavelengths.
[0048] Returning to FIG. 3, the TE optical signals propagating
within the waveguides 315 may be switched by the switching network
320 as described in the '712 application to the various waveguide
gratings 310 of the output arrays 325a-b. The TM optical signals
propagating within the waveguides 340 may be switched to the
various waveguide gratings 310 of the output arrays 325a-b. TE and
TM optical signals separated at the input arrays 305 need not be
switched to the same waveguide grating 310 at the output arrays
325. By virtue of the reduction of TE and TM noise within the
waveguides 315/340 the fidelity of the TE and TM signals recombined
at the output arrays 325a-b is expected to be significantly greater
than would otherwise be the case.
[0049] Turning to FIG. 6, a method 600 of the disclosure is
described in one embodiment. The method 600 may be used to form an
optical device, e.g., the optical devices 200 and 300, including
features described in FIGS. 1-5. The method 600 will be described
without limitation by making reference to the various embodiments
described herein, e.g. by FIGS. 1-5. The steps of the method 600
may be performed in an order other than the illustrated order.
[0050] In a step 610 a input/output optical coupler, e.g. one of
the waveguide gratings 230 or 310, is formed that is configured to
separate a first received optical signal into first and second
signal components. In a step 620 a first waveguide, e.g. one of the
waveguides 205, 210, 255, 260, or 315, is connected to the
input/output optical coupler and configured to propagate the first
signal component via a first propagation mode, e.g. TE. In a step
630 a first waveguide fragment, e.g. the secondary waveguide 120 or
the waveguide fragment 355, is formed adjacent the first waveguide.
The first waveguide fragment is configured to couple light from the
first waveguide that propagates by a second propagation mode, e.g.
TM, that is different from the first propagation mode, e.g. TE.
[0051] In a step 640, the input/output optical coupler is a
waveguide grating. A second waveguide is formed, e.g. waveguide
335, that is connected to the waveguide grating. The second
waveguide is about parallel to the first waveguide, e.g. the
waveguide 315, at the first waveguide grating. The second waveguide
is configured to propagate the second signal component via the
second propagation mode, e.g. TM.
[0052] In a step 650 a third waveguide, e.g. the waveguide 340, is
formed adjacent to the second waveguide, e.g. the waveguide 335.
The third waveguide is configured to couple the second signal
component via the second propagation mode.
[0053] In some embodiments the waveguide grating is one of a
plurality of waveguide gratings, e.g. waveguide gratings 310,
configured to optically couple to a corresponding plurality of
optical cores of a multi-core fiber.
[0054] In a step 660 the input/output optical coupler is a first
waveguide grating. A second waveguide grating, e.g. waveguide
grating 250, is formed that is configured to separate a second
received optical signal into third and fourth signal components. In
a step 670 a third waveguide, e.g. the waveguide 255 or 260, is
connected to the second waveguide grating and is configured to
propagate the third signal component via the first propagation
mode. In a step 680 a second waveguide fragment is formed adjacent
the third waveguide. The second waveguide fragment is configured to
couple light from the third waveguide that propagates by the second
different propagation mode. In a step 690 a 2.times.2 multi-mode
interference coupler, e.g. coupler 270, is connected to the first
and third waveguides.
[0055] In a step 695, wherein the input/output optical coupler is a
waveguide grating, a second waveguide is formed that is connected
to the waveguide grating. The second waveguide is about
perpendicular to the first waveguide at the waveguide grating. The
second waveguide is configured to propagate the second optical
signal via the first propagation mode.
[0056] In some embodiments the input/output optical coupler is a
two-dimensional pattern grating coupler. In some embodiments the
first waveguide is a waveguide of a coherent receiver. In some
embodiments the first propagation mode is TE and the second
propagation mode is TM. In some embodiments the first propagation
mode is about orthogonal to the second propagation mode.
[0057] Those skilled in the art to which this application relates
will appreciate that other and further additions, deletions,
substitutions and modifications may be made to the described
embodiments.
* * * * *